Impossible Smaug the Magnificent

In the Desolation of Smaug Impossible Mode, the Group Ambush states that all players must place a card of cost 8 or higher under this card each time it is defeated. Does that mean that in order to get rid of the card entirely (i.e., having no loot under it), all players must avoid the Group Ambush at once when the final card is taken?

We figured it's called "Impossible" for a reason, so kept adding cards even when the last one was being taken. We only got past it because finally, everyone either avoided the ambush with a Defense card or had no card of cost 8 or higher in their hand at the time and got a Corruption instead.